You need to create a webfont kit for this font using https://www.fontsquirrel.com/tools/webfont-generator and then add that custom style/css at Theme Options > Custom CSS/JS
You will also need to change the path to font files to the absolute URLs using your website address. You can upload the kit font files to your website server through FTP or through WordPress media and use those links in the custom style sheet generated.

Are you sure about the location of the font files as I can not access any of these files from my browser Example: https://www.agioi-pantes.gr/themes/NativeChurch/css/fonts/athonite-webfont.woff2 All shows 404 error. If the location is correct then make sure the file permission is set to make the files accessible. Once you can access these font files using your browser then you can use this font family in any of your class. For example if you want to use this font family for your website body text then you can add custom css: